1. Technical Field
The present invention relates generally to a method and apparatus for measuring physical characteristics of a surface, and, more particularly, to integrated methods and apparatus for measuring thin film thickness and uniformity across the surface of a wafer.
2. Background of the Invention
Various forms of wafers are well known in industry. As is generally known in the electronics industry, wafers are often used to store information and perform functions, particularly in the area of integrated circuits. Examples of common wafers include semiconductor wafers, magnetic disks, optical disks or other work pieces and are generally in the form of a flat, substantially planar disk.
The processes for manufacturing the wafers are also well known. For example, manufacturing a semiconductor wafer might include slicing a wafer from a silicon ingot and then polishing, cleaning, rinsing, and drying the wafer to remove any debris from the surface of the wafer. The polishing, cleaning and planarization may be performed on a typical chemical mechanical planarization (CMP) machine such as the machine shown in FIG. 1. For example, the illustrated CMP machine has various stations for the polishing, cleaning and rinsing of the wafer.
The wafers, in particular, semiconductor wafers, must be planarized and polished to remove excess material and imperfections and to achieve desired electrical properties. Similarly, in the manufacture of optical disks, removal of debris on the surfaces of the disks is important because the debris can cause voids in recorded information on the disks. At each step of the process, as imperfections and debris are reduced or eliminated, the wafers become increasingly more valuable. For example, a typical silicon wafer, prior to any processing, may cost $80-500. However, after the wafer has been planarized, polished and cleaned, it is often worth $20,000-$80,000 or more.
It is also important to measure uniformity and thickness of the wafers at various stages in the CMP process in order to ensure the quality of the finished wafers. More efficient and rapid acquisition of information is needed to control the CMP process, and the ability to use the information to optimize the performance of a particular polish tool, the ability to minimize rework and prevent generation of scrap wafers due to mis-processing is becoming increasingly important. Thus, CMP process control requires frequent, if not continual measurement to verify that the desired material removal is occurring and to ensure that acceptable wafer uniformity exists.
Metrology devices are generally required to perform the wafer measurements and are often performed using optical means. The metrology devices typically operate by directing light at the surface to be measured and measure the characteristics (e.g., intensity, angle of reflection, defraction, scattering, etc.) of the light reflected from the surface. The characteristics are then used to calculate various properties of the thin film covering the wafer surface, such as, the index of refraction, extinction coefficient, and thickness of the thin film. Exemplary of existing metrology units include the Nanospec 9000(trademark) manufactured by Nanometrics(trademark), the Thermawave 3260 manufactured by ThermaWave, and the UV1050 manufactured by KLA-Tencor.
Typically, metrology devices such as these comprise separate and distinct stations apart from the main CMP tool. This requires that the wafers be transported to the metrology station for measurement. However, use of separate stations has various undesirable consequences when implemented in CMP processes. For example, it is often economically desirable to increase the speed of the wafer manufacturing operations in order to increase throughput. Stated otherwise, wafer manufacturers aspire to process a greater number of wafers in a smaller amount of time and at a lower cost. Thus, the speed of the device transporting the wafers from station to station (e.g., from polishing stations to metrology stations) might be increased. Nonetheless, the additional transport steps to the metrology stations still lowers the overall throughput of wafers irrespective of speed increases.
Further, every time a wafer is xe2x80x9chandled,xe2x80x9d for example, by the end effector (the device which contacts the wafer during transport), there is an increased possibility for introducing additional defects to the wafer. Wafer manufacturers attempt to reduce the effects of additional handling steps by sampling wafers. That is, rather than measuring every wafer processed, one wafer from a predetermined number of wafers (a sample) is measured and statistical methods are used to determine whether the wafers which have not been measured are within tolerances. Unfortunately, sampling the wafers still slows the manufacturing process to some extent and leaves the risk that some wafers which are not measured do not fall within desired tolerances.
Thus, integrating existing metrology devices with CMP equipment so that fewer wafer transport steps are required has been considered. However, until the present invention, so doing has not previously been possible. This is at least in part because using existing metrology devices, the mechanical integration of the device with existing CMP equipment is complex, costly and time consuming. As mentioned above, additional wafer transporting equipment is thus necessary, throughput is reduced and the potential of adding defects is increased.
Accordingly, because increased throughput of processed wafers is generally desired and because the risk of adding imperfections and defects to the wafers as the amount of handling is increased, methods and apparatus to measure the uniformity and thickness of the wafers in a manner which minimizes any need for additional processing steps are desirable.
The present invention provides methods and apparatus for presenting a wafer to a metrology device for measuring thin film characteristics. In accordance with one embodiment of the present invention, a metrology device is integrated with a CMP tool such that various wafer measurements can be automated while minimizing additional handling of wafers processed by the CMP tool. For example, a metrology device may be integrated between stations within a CMP process tool. As a wafer transfer device moves a wafer from an index load cup of the CMP tool or as wafers are drawn from a spinner, the transfer device and wafer may pass by or over the metrology device. As the wafers pass across the device, moving from station to station, the device measures finite points on the wafer surface. The measurements are used to calculate various physical characteristics of the wafer surface. For example, uniformity and mean film thickness of the wafer may be determined. Thus, as the wafer transfer device moves wafers from station to station, the wafer is presented to the metrology device with no substantial decrease in the throughput of wafers.
In accordance with another aspect of the present invention, the metrology device comprises a light source provided from multiple emission points configured in an array or multiple arrays. When using multiple output arrays, the arrays are preferably arranged side by side and substantially parallel to allow the measurement of multiple points on the wafer surface increasing accuracy of the measurement of the wafer surface.
In accordance with yet another aspect of the present invention, a wafer location means is provided to track the position of the wafer passing over the wafer measurement device. Preferably, the tracking device comprises a curtain comprising a light beam and photoelectric cell detects when the wafer is entering and/or leaving the measuring device and suitably enables the tracking of the location of the wafer. Combining multiple sensors, and means for sensing the wafer""s position, preferably independent from the CMP process tool""s wafer handling system, thereby increase the speed of measurement and ease integration of the device into a process tool.